This invention relates to methods and apparatus for protecting nuts and protruding threads of bolts, and is particularly suitable for protecting nuts and threads in equipment exposed to corrosive environments.
Most mechanical equipment is assembled with nuts and bolts, which enables the equipment to be disassembled for maintenance and repair. The exposed threads of machines located within manufacturing plants generally do not corrode rapidly, and therefore maintenance personnel can easily unthread the required nuts in order to disassemble the machines. In equipment located outdoors or equipment exposed to highly corrosive environments, however, the exposed threads often corrode to the extent that the nuts cannot be removed or can only be removed with extreme difficulty.
In the petroleum industry, for instance, large quantities of equipment, such as valves and flanges, are located outdoors or are otherwise susceptible to highly corrosive environments. Such equipment may require disassembly only after years of service, by which time the exposed threads of bolts are so corroded that the nuts cannot be removed. Countless man hours have been spent trying to remove nuts from such equipment. Often the nuts can be broken free and unthreaded a short distance, but the exposed threads are so damaged or corroded that the nut cannot be further removed. In many cases, the removal operation severely damages the nut or bolt; in other instances, hydraulically powered hacksaws are used to cut through the bolt and nut so that the equipment can be disassembled. Corrosion of the exposed threads in this equipment results in a tremendous loss of time and material, and is a continual problem for maintenance personnel.
Many attempts have been made to alleviate this problem. Nuts and protruding threads have been coated with various lubricants in an attempt to inhibit corrosion. One problem is that these lubricants generally tend to wear away and do not remain on the threads for a sufficient length of time. Thus, the threads must be recoated on a regular basis, which is costly and time consuming.
Other coatings have been tried, such as lead coatings and special paints, but these coatings are expensive and often do not result in sufficient corrosion inhibition for the entire area of the thread. If someone or something comes into contact with these coatings, some of the coating may rub off, so that a portion of the thread is exposed and therefore corrodes. Unfortunately, corrosion of one portion of the thread is almost as deleterious as corrosion of the entire exposed thread, since corrosion even at one portion my result in the inability to unthread the nut from the stud or bolt.
Other persons have attempted to solve the problem with special cap nuts. Cap nuts such as are shown in U.S. Pat. Nos. 1,727,590 and 2,095,289 are not practical in many applications because of their high cost and variations in the length of the exposed bolt after the nut has been properly tightened.
Various types of cup-shaped thread protectors have been invented. The thread protectors illustrated in U.S. Pat. Nos. 972,140; 1,254,514; 1,630,584; 2,456,234; 2,551,834; 2,726,009; and 3,135,558 offer some protection to the exposed threads from the environment, but do not adequately inhibit corrosion at the thread. Corrosion of the threads may be reduced by merely sliding a cap over the threads, but moisture and corrosive elements will continue to attack the threads.
Some of the devices illustrated in the above patents utilize a protective cap and additionally attempt to seal the exposed threads from the environment. Such devices, however, have not proven to be commercially acceptable in many applications. In some instances, the more elaborate devices are not economically justified. Also, many of these devices are difficult or time consuming to install, and this is particularly true for those devices which attempt to seal the otherwise exposed threads from the environment.
Some of these patents disclose utilizing a lubricant such as grease in the interior of the cap to inhibit corrosion. Since it is frequently difficult, if not impossible, to introduce the protective lubricant into the interior of the cap after the cap has been installed, these devices have inherent disadvantages. A protective fluid having a high viscosity, e.g. grease, has been used in order to enhance retention of the fluid within the cap while the cap is installed. However, if the cap is to be mounted on a stud protruding vertically upward, even a high viscosity fluid may not be retained within the cap while the cap is being installed. Additional problems are encounted because the lubricant is physically handled by the cap installer. If one inserts too much lubricant within the cap, the protective cap may not thereafter be fully and properly positioned on the stud. An insufficient amount of lubricant, however, may not adequately protect the threads from corrosion.
Even after the cap with some protective lubricant has been installed, the prior art devices do not provide continued satisfactory protection for the threads. For example, it is difficult, if not impossible, for someone to subsequently determine whether lubricant was inserted before the cup-type thread protector was installed. Also, one cannot easily ascertain whether the lubricant has leaked from the thread protector. If lubricant below the thread protector suggests that lubricant may have leaked from the thread protector, the thread protector must be removed and reinstalled in order to bring additional lubricant into contact with the threads. Outdoor temperature variations may cause a pressure increase within the prior art caps, and pressure may increase within the cap to the extent that the cap is forced off the threads. Finally, the devices illustrated in the above patents do not satisfactorily protect the threads from corrosion, since the mere presence of a lubricant within the cup-shaped thread protector will not adequately inhibit corrosion of the threads.
An additional problem exists for protecting the nut. The threads in contact with the nut generally do not easily corrode since they are protected from the environment by the nut. The outer portion of the nut, however, is subject to corrosion. Corrosion may not structurally weaken the nut, but may inhibit the use of a proper sized wrench to remove the nut. Also, petroleum equipment such as valves and flanges are frequently painted in an attempt to inhibit corrosion of the equipment. The continued build-up of paint on the nut may likewise inhibit the use of a proper-sized wrench when the nut is to be removed.
If the proper-sized wrench is to be utilized, it may first be necessary to grind or chip away at the corrosion or paint build-up. If the corrosion or paint build-up on the nut is severe, maintenance personnel may opt for attempting to remove the nut with an oversized wrench. This procedure may, of course, damage the nut so that it can no longer be removed. Even worse, personnel using an improper wrench risk injury because of unsafe practices.
Some of the protectors in the above-cited patents also cover the nut. For instance, U.S. Pat. Nos. 2,456,234 and 2,726,009 disclose protectors which cover both the threads and the nut.
These nut protectors may function satisfactorily as a paint shield to cover the nut, and may even provide sufficient corrosion protection for the external surface of the nut. The problem, however, is that the nut protectors in the prior art patents are expensive to produce. Additionally, the prior art nut protectors are difficult or time consuming to properly install over the nut.
The problems and disadvantages of the prior art techniques, coating materials, and thread and nut protectors are such that these methods, materials and devices are not widely accepted in many applications. In the petroleum industry, for instance, nuts and exposed threads on equipment located outdoors are frequently not protected from the corrosive environment. If the equipment is especially susceptible to corrosion, the nuts and bolts may be fabricated from special metals. For instance, petroleum equipment handling high sulfur crude oil may corrode at an accelerated rate due to the presence of sulfur. The bolts and nuts of such equipment may be fabricated from stainless steel or a chrome alloy. Generally, this technique will adequately inhibit corrosion of the exposed threads and nuts, but the cost of the special-material nuts and bolts severely limits the use of this technique.
These problems and disadvantages of the prior art are overcome with the present invention. Novel methods and apparatus are hereinafter provided for protecting nuts and protruding threads of bolts or studs.